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Michael; Sorry to be late responding, I just saw your post. I also have a late serial number 48 with the C-90 and I had your symptoms when I first got my plane. Sure enough the motor had been top ended and "improved" with drilled rocker arms. Going back to the late-model intake rockers cured my oil in the intake/carb box. So...I would say that Randy has the measure of it
Steve Rose
I was about to send my rocker arms in to Aicraft Specialties and noticed I have the same issue. All the rocker arms are drilled. At around $500 each, it's too bad the hole can't just be plugged.
I contacted Todd at Smokin' Rivets and he got me some exchanged overhauled rockers cheap, make sure you check the pushrod lengths on valve lash for overhauled rockers.